This invention relates generally to processes for incorporating polystyrene in block copolymers and to the compositions produced thereby. More specifically, the invention relates to a process for producing a vinyl aromatic hydrocarbon block terminated block polymer, such as SBS, interpolymerized with a polymer formed from vinyl aromatic hydrocarbon monomer contributed unites, such as polystyrene; and a product having a substantially improved Gardner impact strength, produced by such a process.
The prior art has long strived to improve the physical properties of styrenic polymers. For instance, U.S. Pat. No. 4,267,283 to Whitehead teaches a two-component graft copolymer composition having improved toughness. The first graft polymer component is disclosed as consisting essentially of: from about 8.0 to about 16.0 parts by weight of a mixture of an ABA block copolymer and an Axe2x80x2Bxe2x80x2Axe2x80x2 tapered block copolymer in a weight ratio of the ABA copolymer to the Axe2x80x2Bxe2x80x2Axe2x80x2 copolymer of between about 25:75 and about 75:25. Each A segment is an essentially pure polymer block of styrene having a number average molecular weight of between about 14,000 and about 18,000. Each B segment is an essentially pure polymer block of butadiene having a number average molecular weight of between about 60,000 and about 80,000; the B block having a glass transition temperature of about xe2x88x92105xc2x0 C.+/xe2x88x925xc2x0 C. The weight ratio of total A to B being between about 1:1.8 and about 1:2.7. Each Axe2x80x2 segment represents essentially polymerized styrene. The balance of the Axe2x80x2 segment is polymerized butadiene. The Bxe2x80x2 segment represents essentially polymerized butadiene. The balance of the Bxe2x80x2 segment is polymerized styrene. The weight ratio of total Axe2x80x2 to Bxe2x80x2 being from about 1:2.6 to about 1:3.6, the number average molecular weight of said Axe2x80x2Bxe2x80x2Axe2x80x2 block copolymers being between about 400,000 and about 660,000. The Bxe2x80x2 block has a glass transition temperature of about xe2x88x9290xc2x0 C.+/xe2x88x925xc2x0 C. The second graft component consist essentially of from about 92.0 to about 84.0 parts by weight of monomeric styrene polymerized in the presence of the ABA and Axe2x80x2Bxe2x80x2Axe2x80x2 copolymers.
Similarly, U.S. Pat. No. 3,954,696 to Roest, teaches a process for the preparation of block copolymers of the general formula Axe2x80x94Bxe2x80x94C. This process includes the steps of polymerizing at least one monomer to form a living polymer block A; adding a further monomer and continuing polymerization to form polymer block B bound to polymer block A, and continuing polymerization while adding at least one monomer to form terminal polymer block C, so as to produce an Axe2x80x94Bxe2x80x94C block copolymer. Each of the polymer blocks A and C consist of either a non-elastomer homopolymer or copolymer having a glass transition temperature over 25xc2x0 C. And a number average molecular weight between 200 and 100,000. The polymer block B consists of a conjugated diene, derived from preferably 1,3-butadiene or isoprene, having a glass transition temperature below xe2x88x9210xc2x0 C. and a number average molecular weight between 25,000 and 1,000,000. The contaminants contained in the monomers forming blocks A and C are thereafter deactivated. As his improvement over the prior are, Roest includes contaminants in the conjugated diene monomer forming polymer bock B, that have not been deactivated and that are capable of killing 1-50% of the living polymer block A upon introduction of conjugated diene monomer to the reaction mass. Each of the polymer blocks A and C are disclosed as consisting of a non-elastomeric polymer block having a glass transition point over 50xc2x0 C. and a number average molecular weight between 500 and 50,000. The polymer block B is disclosed as consisting of an elastomeric polymer block having a glass transition point below xe2x88x9225xc2x0 C. and a number average molecular weight between 50,000 and 500,000. At least one of polymer blocks A and C is derived from a monovinyl aromatic hydrocarbon.
U.S. Pat. No. 3,265,765 to Holden et al, discloses an unvulcanized elastomeric block copolymer having the general configuration Axe2x80x94Bxe2x80x94A. Holden discloses that block A is an independently selected non-elastomeric monovinyl aromatic hydrocarbon polymer block having an average molecular weight of 2,000-100,000 and a glass transition temperature above about 25xc2x0 C. The total block A content being 10-50% by weight of the copolymer. Block B is an elastomeric conjugated diene polymer block having an average molecular weight between about 25,000 and 1,000,000 and a glass transition temperature below about 10xc2x0 C. The copolymer is prepared with a lithium-based catalyst and has a tensile strength at 23xc2x0 C., in excess of about 1400 pounds per square inch.
In yet another similar U.S. Pat. No. 3,231,635 to Holden et al., an unvulcanized elastomeric block copolymer having the general configuration Axe2x80x94Bxe2x80x94A is disclosed. Block A is an independently selected non-elastomeric monovinyl aromatic hydrocarbon polymer block having an average molecular weight of 2,000-100,000 and a glass transition temperature above about 25xc2x0 C. The total block A content being 10-50% by weight of the copolymer. Block B is an elastomeric conjugated diene polymer block having an average molecular weight between about 25,000 and 1,000,000 and a glass transition temperature below about 10xc2x0 C. The copolymer is prepared with a lithium-based catalyst and has a tensile strength at 23xc2x0 C., in excess of about 1400 pounds per square inch.
U.S. Pat. No. 3,239,478 to Harlan, teaches an adhesive composition that comprises components. The first component of the composition comprises 100 parts by weight of a block copolymer having the general configuration Axe2x80x94Bxe2x80x94A. Each A block is an independently selected polymer block of a vinyl arene. The average molecular weight of each A block is between about 5,000 and about 125,000. The B block is a polymer block of a conjugated diene. The average molecular weight of the B block is between about 15,000 and about 250,000. The total of the A blocks is less than about 80% by weight of the copolymer. The second component of the composition comprises about 25-300 parts by weight of a tackifying resin. Finally, the third component of the composition comprises 5-200 parts by weight of an extender oil. The oil is substantially compatible with homopolymers of the conjugated diene.
Finally, U.S. Pat. No. 3,149,182 to Porter teaches a process for preparing an elastomeric three component block copolymer. The copolymer comprises the first step of: contacting a monomer of the group consisting of diolefins containing from 4 to 10 carbon atoms, mono alkenyl-substituted aromatic hydrocarbons and mono-alkenyl-substituted pryidine compounds with a hydrocarbon lithium compound in an inert atmosphere and under substantially anhydrous conditions until the unpolymerized monomer in the reaction mixture is consumed. Next, without further treating the reaction, adding a monomer of the above group which is similar to that used in the initial reaction. Thereafter, continuing the polymerization under the above conditions until the dissimilar monomer has been polymerized. Next, without further treatment of the reaction mixture, adding a third monomer which is different from the aforementioned dissimilar monomer and selected from the above group of monomers. Finally, the polymerization is continued under the aforedescribed conditions until the third monomer has been completely consumed. At least one of the foregoing monomers is a diolefin.
Despite the foregoing prior art, there nonetheless exists a long felt need for a process for predicably producing styrenic polymers exhibiting high Gardner impact strengths in excess of at least 60 fl lb/in, as well as such other polymers and articles produce therefrom.
Surprisingly, the instant inventors have discovered that by merely manipulating the weight proportions of the respective polymers of the interpolymer mix, dramatic increases in the Gardner Impact Strength of the product may be achieved.
Polystyrene is a well-known thermoplastic material finding a wide variety of uses. It is often added to polymers including block copolymers to increase the mold flow characteristic of the polymer, thus preventing the polymer from sticking to the injection molder cavity. Heretofore, polystyrene has been blended with block copolymers to increase the processability of the block copolymer. U.S. Pat. No. 4,308,358 to Miller, discloses a process for making high impact polystyrene comprising mixing, at an elevated temperature, an AB block copolymer and a styrene polymer. This blending process creates disadvantageous properties in the blend, namely the impact strength of the block copolymer is severely reduced upon the addition of as low as 1.5% by weight of crystal polystyrene to the block copolymer. While not wishing to be bound by any particular theory, Applicants believe that the lower impact strength resulting from the blending of polystyrene and block copolymer is due to the different molecular weights and physical properties of the components thereby causing phase separation to occur in the resulting product. The poor interphase adhesion characteristic of highly incompatible blends usually results in very poor mechanical properties, e.g., tensile strength, elongation and impact strength.
It is therefore an object of the present invention to provide a process for producing an interpolymer of polystyrene and a block copolymer exhibiting good mechanical properties. It is a further object of this invention to provide polystyrene and block copolymer products exhibiting high impact strength.
In contrast to the foregoing prior art, the instant invention provides a process for interpolymerizing a blend of a vinyl aromatic hydrocarbon polymer and copolymer product is disclosed. The process includes the following steps:
(a) forming in a suitable diluent a block polymer precursor having a living end and having at least one polymeric block containing conjugated diene monomer contributed units in the presence of an anionic initiator;
(b) thereafter adding to the block polymer precursor a charge of vinyl aromatic hydrocarbon monomer and an additional amount of anionic initiator to simultaneously form (1) a block polymer having a terminal block, formed from the vinyl aromatic hydrocarbon monomer, attached to the block polymer precursor and (2) poly(vinyl aromatic hydrocarbon) interpolymerized with the block polymer of (1).
The practice of this process produces a vinyl aromatic hydrocarbon block terminated block polymer, such as SBS, interpolymerized with a polymer formed from vinyl aromatic hydrocarbon monomer contributed units, such as polystyrene.
Surprisingly, the instant inventors have discovered that by merely manipulating the weight proportions of the respective block polymer and polyvinyl aromatic hydrocarbon polymer of the interpolymer mix, dramatic increases of in excess of about 60 ft lb/in to at least about 200 ft lb/in of the Gardner Impact Strength of the final product may be achieved.